This invention relates to an isomerization process for conversion of cyclic terpenes into less cyclic hydrocarbons. More particularly, this invention is directed to an improved thermal isomerization process whereby certain bicyclic terpenes-i.e. pinane, alpha-pinene and beta-pinene are converted in high selectivity into open-chain or acyclic isomers thereof-e.g., allo-ocimene, dihydromyrcene and myrcene-by high temperature reaction in the presence of a solid material comprising a metal and/or metal oxide supported on a carrier having a specific surface area in the range of about 0.01 to about 20 m.sup.2 /g. The acyclic or open-chain hydrocarbon products of this isomerization are useful as base materials or intermediates in the synthesis of a variety of aroma or perfume chemicals e.g., geraniol, citral, citronellol, linalool, ionones and the like.
It is well known that bicyclic terpenes such as beta-pinene can be thermally isomerized or pyrolyzed to afford varying quantities of acyclic or open-chain isomers. Depending on the bicyclic starting material employed and the reaction conditions selected, the isomerization product typically contains significant amounts of one or more monocyclic hydrocarbons e.g. limonene, in addition to the desired acyclic products. Examples of such thermal isomerization processes include the process described in British Pat. No. 910,879 directed to the preparation of 3,7-dimethyloctadi-1,6-ene (dihydromyrcene) from pinane by vapor phase rearrangement at 360.degree. to 555.degree. C. In the process of the aforementioned British patent, the yield to dihydromyrcene is enhanced by controlling the reactant (pinane) conversion at low levels while maintaining high heat transfer in the reaction tube through the use of copper filler bodies. Another thermal isomerization process for bicyclic terpenes is described in British Pat. No. 1,007,339. In this process, terpene compounds, such as pinane and pinene, are partially pyrolyzed to certain acyclic isomers e.g., ocimene, using short reaction times and high heat transfer rates provided by a reactor containing a heated element immersed in an environment of liquid reactant.
In the foregoing examples and other such thermal isomerization processes, effective heat transfer is quite important due to the relatively high temperatures required for good reactant conversion and the unstable nature of the unsaturated acyclic product obtained at reaction conditions. Thus, while high temperatures are necessary for high conversion, such high temperatures can be tolerated only for rather short residence times without causing considerable reduction in the selectivity with which the bicyclic reactants are converted to the desired open-chain products In this regard, certain shortcomings can be perceived for the thermal isomerization processes previously employed. Due to the relatively high reaction temperatures, carbon is usually deposited during the reaction on the reactor walls and/or on the heat-transferring filler bodies. The removal of the carbon deposits is often difficult and time consuming. Further, while a possibility exists for reducing the severity of the reaction and/or deposition of carbon through the use of a catalyst, previous efforts using catalysts in the thermal isomerization of unsaturated bicyclic hydrocarbons such as alpha-or beta-pinene seem to be directed more towards enhancing the yields or monocyclic products rather than the desired acyclic isomers. For example, U.S. Pat. No. 3,270,075 teaches that the yield of the cyclic product limonene from the cracking of alpha-or beta-pinene may be increased by carrying out the cracking reaction in the presence of a zeolitic metal aluminosilicate catalyst.